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Lin, Shenyu (2016): Anatomical and histomorphometric observations on nerve transfer in the distal forearm for the reconstruction of hand function. Dissertation, LMU München: Faculty of Medicine



In the past century, significant understanding in the field of peripheral nerve surgery has been made with the increasing advances of microsurgical techniques and knowledge of topography of peripheral nerves as well as the cellular and molecular events. As our understanding of nerve injury and repair increases, new techniques of nerve repair including nerve autograft, nerve allograft, tendon transfers and nerve transfers have been performed in the clinic. Although autografting is still the gold standard of nerve repair when possible, nerve transfers have gained great popularity among surgeons especially in the distal forearm for wrist and hand functional reconstruction. The most frequently distal nerve transfer is the transfer of the AIN to the DBUN for intrinsic hand reconstruction. Specific successful nerve transfer of the AIN to the DBUN has stimulated us to transfer the AIN to the TBMN to reconstruct the thenar muscle function and transfer the DBCUN to the SBRN or the SMN for sensory reconstruction. As previously reported, the AIN can be sacrificed because the loss of pronation function in the forearm can be compensated by the pronator teres muscle and the DCBUN can be cut because the medial dorsal side of the hand is a non-critical area. This feature of the AIN and the DCBUN allows us to use them as donor nerves which meet the technical point of nerve transfer in the upper extremity ‘donor distal, recipient proximal’. Therefore we cut the AIN at the proximal border of the pronator quadratus muscle and the DCBUN before the first bifurcation in order to maximize the axon number and decrease the regeneration distance. For the recipient nerve, we transected the SMN and the TBMN proximally enough so that they can be mobilized to allow a tension-free coaptation. Moreover, divided proximally can avoid necessity for nerve grafting as well as axon misdirection, which could substantially downgrade the functional recovery. There are two sides for everything and that certainly is time for nerve transfers as well. The major drawback of nerve transfers is sacrificing a viable nerve for an injured one, losing or diminishing the function of a muscle for more important functions, or to sacrifice a non-critical area’s sensation for critical area’s sensation. For surgeons, we need to take a risk-to-benefit ratio into consideration before we perform the operation. Therefore the anatomic and histomorphomoetric data of the nerves are crucial for us when we tailor the plan for the patient individually. In keeping with this, anatomical and histomorphometric data of nerve transfers including the motor nerve transfer from the AIN to the TBMN and sensory transfer from the DCBUN to the SBRN and the SMN were tested and documented in our study, which provided a basis for managing the peripheral nerve lesions in the hand. The nerve transfers were performed in 15 fresh cadaver specimens.The overall length of the forearm was documented 252 ± 6.0 mm from the lateral epicondyle of the humerus to the styloid process of the radius. Nerve samples were transected from the distal side of the donor nerve and proximal side of the recipient nerve at coaptation site for histomorphometric observation. The tension-free coapation sites were measured with relation to the anatomical landmarks. In the motor nerve transfer study, our anatomic data indicate that the AIN is a suitable donor nerve for the TBMN. Donor nerve and recipient nerve can be coapated in a tension-free manner after the SMN and the TBMN were proximally divided and mobilized over a length of 97 ± 4.0 mm to reach the coaptation site. It appears that an optimal site for coaptation of the AIN and the TBMN is at the proximal edge of the PQ which was recorded as 202 ± 4 mm distal from the lateral epicondyle of the humerus. Comparison of the AIN to the TBMN, the AIN has significantly less density, smaller diameter, fascicle and nerve cross-sectional area, but a comparable fascicle number. The axon ratio of the AIN to the TBMN is 1:3.7 which was slightly less than the commonly accepted successful threshold 1:3, but multivariate analyses have shown that 3-4 collaterals can be developed by one axon; hence we think that the AIN is a suitable donor nerve for the TBMN. In addition to the directly end-to-end suture, the SETS AIN-to-DBUN transfer has been described with excellent result in an incomplete injury of the ulnar nerve. This clinical scenario provides us a new choice for the reconstruction of thenar muscle by the SETS AIN-to-TBMN transfer. In the sensory nerve transfer, our anatomic data show that the DCBUN was a suitable donor nerve for the SMN and the SBRN. In order to maximize the axon number of the donor nerve, the DCBUN was cut prior to its first bifurcation.The SBRN was transected prior to its first bifurcation, which made the donor nerve axons grow into the whole recipient to supply the lateral dorsal hand. The SMN was separated from the TBMN over a distance of 82 ± 6 mm which ensured a tension-free copatation with the DCBUN. Histomorphometric data indicate that there were no significant differences (p < 0.05) between donor and recipient in terms of total fascicle number, fascicle area, nerve diameter, nerve area and axons. Based on these results, the DCBUN can be accepted as a suitable donor nerve for sensation restoration in the hand. In the past decade, accompanying with the development of nerve reconstruction from nerve grafts to nerve transfers, the difficulties and possibilities of motor or sensory nerve transfers were concern by many peripheral nerve surgeons. One of the greatest concerns of surgeons was the nerve reeducation after operation. Clinically, many transfers are performed with little or even with no training. But it is known that rehabilitation is helpful by recruiting the donor muscle groups preoperatively and repeating these activities until reinnervation is recognized. In keeping with this, early rehabilitation of the motor and sensory functions should be encouraged for the patient. With the increasing understanding of the nerve topography and redundancy as well as the advances of the basic science and clinical research, potential nerve reconstructions with end-to-end, end-to-side and reverse end-to-side transfers will continue to be expanded and become available.